The present invention relates to a sensor comprising an array of detectors and in particular the invention relates to a sensor of this type for use in security or surveillance applications.
Presently available sensors for use in security or surveillance generally fall into two categories. There are simple passive sensors such as passive infrared sensors which detect movement of objects and generate corresponding warning or alarm signals and there are more complex surveillance devices such as closed circuit television cameras which employ a multiplicity of detector elements to provide an image of the scene being monitored.
The present invention is based on the realisation that it would be useful to detect the absence of movement of objects such as a person falling asleep when it is necessary that they are alert and mobile or a person not getting up after a fall.
The invention provides a sensor comprising a linear or two-dimensional array of detectors, optical collection means arranged so that spatial information from a scene is focused onto the array, read out means for monitoring signals from the detectors, means responsive to the said signals for identifying the entry of an object into a first selected area of the scene and means for generating a warning or alarm signal after a first predetermined period of time during which there is no movement of the body within the first selected area.
The selected area could be the entire scene which is focused onto the array, but it is more likely that it would be part of the scene. The size of the object could be such that it occupies any part of the scene and consequently any number of elements of the array image.
The sensor of the invention would be particularly useful for example in a security environment where it could be used to detect when an object has been moved in front of the sensor such that it obscures the field of view of the sensor; an alarm could be raised if the field of view remains obscured for more than a predetermined length of time. It would also be particularly useful, for example, at the home of an elderly or infirm person; in this case it could be used to detect the person falling over and not getting up again or to detect a person sitting in a chair or lying on a bed for an unusually long period of time. Thus a sensor according to the invention could be used to survey an area in the home including the person""s bed; it could detect the person going to bed and raise the alarm if the person is still on the bed, motionless, after a predetermined length of time.
A sensor according to the invention could be used to raise an alarm after no movement has been detected in a number of different locations within the scene. Thus, the preferred sensor according to the invention includes means for identifying the entry of an object into a second area of the scene and means for generating a warning signal after a second predetermined period of time during which there is no movement of the object within the second selected area. The second period of time may be different from the first period of time. In fact, the number of different selected areas monitored by the sensor according to the invention may be more than two and each area may have a different allocated predetermined period of time. Thus, for example, a sensor according to the invention could be programmed to give a warning or alarm after, say, 8 hours on the bed, 3 hours on the armchair or 10 minutes on the floor, the latter being to detect a person who has fallen over and not got up again. The sensor according to the invention can, of course, also be used to detect intruders if programmed accordingly.
A sensor according to the invention might be provided with a means for programming xe2x80x98in situxe2x80x99 according to the layout of the particular space being monitored, or it may have the capability of self-learning.
An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: